The present invention relates to a smoothing circuit for a switching power supply used to charge a battery and so on, which includes a combination of a reactor and a capacitor.
In a smoothing circuit for a switching power supply comprising a combination of a reactor and a capacitor, a higher switching frequency is used, and a film capacitor or a ceramic capacitor is used instead of an aluminum electrolytic capacitor in order to increase the life expectancy and reduce the size. FIG. 10 is a connection diagram showing an embodiment of a DC-DC converter section of a conventional switching power supply. In this figure, 1 is a transformer; 2 is a switching element; 3 and 4 are diodes; 5 is a reactor; 6 is a capacitor; and 7 is a resistor. A control circuit for driving the switching element 2 is omitted. When a film capacitor or a ceramic capacitor is used as the capacitor 6, the control system for the DC-DC converter section becomes unstable due to a small equivalent resistance of this capacitor. The resistor 7 having a small resistance value is connected in series to the capacitor in order to stabilize the control system.
FIG. 11 shows waveforms at the components in FIG. 10. A triangular ripple current with an inter-peak current .DELTA.I such as that shown in FIG. 11 flows through the reactor 5 and capacitor 6 to generate at both ends of the resistor 7 (its resistance value is defined as R.sub.7) a triangular ripple voltage with an inter-peak voltage .DELTA.I.times.R.sub.7, which constitutes an output ripple voltage.
Since the output ripple voltage is a triangular wave, it contains a relatively large amount of harmonic components with a relatively high frequency to create magnetic noise corresponding to an AM frequency in a cable connected to the output terminal of the switching power supply, thereby causing radio disturbance.
To eliminate this noise, an LC filter can be additionally connected to the output terminal. Though this can reduce a noise, however, it makes the operation of the control system for the DC-DC converter section unstable.
Also, a ripple attenuator module is commercially available for eliminating this noise. FIG. 12 shows the principle of the operation of a commercially available ripple attenuator module. In this figure, 40 and 41 are DC cut circuits; 42 and 45 are differential amplifiers; 43 is a MOSFET; and 44 is a reference voltage. Ripple noises from the DC cut circuits 40 and 41 are controlled by using the differential amplifier 42 and MOSFET 43, and a direct current is controlled by using the differential amplifier 45 and MOSFET 43 in such a way that the voltage V.sub.DS between a drain and a source of the MOSFET 43 becomes 0.36 V, thereby reducing the output noise. A loss corresponding to output current.times.0.36 V, however, unavoidably occurs, so the efficiency decreases by 10% or more if the output voltage is 3 V. In addition, if the output current is approximately 100 A, the loss will be 100 A.times.0.36 V=36 W, thereby requiring larger radiating fins. In addition, since a frequency of 1 MHz or greater can not be attenuated, in order to prevent an AM radio from being affected by radio disturbance caused by the noise generated from the output cable, the cable must be shielded.
In view of these problems, it is an object of this invention to provide an output noise reduction device that operates stably without reducing efficiency.